This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A115398 #21 Nov 27 2023 19:53:28 %S A115398 3,5,11,12,19,28,61,64,79,92,101,104,199,356,596,692,1709,3539,3824 %N A115398 Numbers k such that both k^2+1 and 2^k + 1 are semiprimes. %C A115398 Intersection of A085722 and A092559. %e A115398 11 is a term because 11^2 + 1 = 122 = 2*61 (semiprime) and 2^11 + 1 = 2049 = 3*683 (semiprime). %t A115398 Select[Range[700],PrimeOmega[#^2+1]==PrimeOmega[2^#+1]==2&] (* _Harvey P. Dale_, Apr 14 2019 *) %o A115398 (PARI) isok(n) = (bigomega(n^2+1) == 2) && (bigomega(2^n+1) == 2); \\ _Michel Marcus_, Oct 10 2013 %o A115398 (Magma) IsSemiprime:=func<n | &+[k[2]: k in Factorization(n)] eq 2>; [n: n in [2..700] | IsSemiprime(n^2+1) and IsSemiprime(2^n+1)]; // _Vincenzo Librandi_, Oct 10 2013 %Y A115398 Cf. A085722, A092559. %K A115398 nonn,more %O A115398 1,1 %A A115398 _Zak Seidov_, Mar 08 2006 %E A115398 a(17)-a(19) from _Robert Israel_, Nov 27 2023